Implantable biomaterials, especially collagen-based biomaterials, require sterilization and most often storage before use. Generally there are two broad classes of implantable collagen-based biomaterials: (1) natural tissue and (2) chemically cross-linked tissue. Thus, depending upon the type of collagen-based biomaterial and whether or not cross-linking has taken place there is a need for a means of sterilizing the tissue as well as storing tissue once it has been sterilized.
Chemical cross-linked collagen-based biomaterials such as cardiovascular patches, heart valves, matrices and arteries are usually sterilized after cross-linking and stored in a sterile solution until implantation. Several sterilization methods for chemical cross-linked collagen-based biomaterials have been tested and implemented over the past three to four decades including gamma irradiation, UV irradiation and a variety of chemical agents. Although most of these sterilization methods are efficient in preventing contamination, adverse effects such as structural damage (cleaving of peptide bonds) and tissue degeneration (reduction in tensile strength) has made a number of these methods less appealing for industrial application.
For example, collagen-based biomaterials cross-linked with glutaraldehyde can become chemically unstable when exposed to alcohol-based sterilisation solutions due to the interaction of the alcohol with residual and unbound glutaraldehyde present in the tissue. Unstable hemiacetyls are formed when an alcohol reacts with an aldehyde. These unstable hemiacetyls have the capacity to react with alcohol to form an acetyl, which can dissociate to form an aldehyde and an alcohol.
Thus, at present, the majority of manufacturers of collagen-based biomaterials prefer the use of glutaraldehyde-formaldehyde combinations for chemical cross-linking and non-aldehyde agents for sterilization. One such non-aldehyde agent is ethylene oxide (oxirane) gas, which has been used to sterilize mechanical heart valves for many years. Ethylene oxide gas has also been used to sterilize a variety of medical equipment, disposable items and mechanical heart valves.
Once the collagen-based biomaterial has been sterilized it is generally stored for a period of time before implantation. Mid- to long-term storage of collagen-based biomaterials requires adequate protection from contamination in a physiologically, stable solution. Although most of the commercially available collagen-based biomaterials are still stored in aldehyde-based solutions, adverse effects such as calcification and fibrosis are well known.
Since the 1970's propylene oxide has been used as a sterilizing agent (see, for example, Hart & Brown, 1974, Appl Microbiol, December p.1069-1070; Brown & Ng, 1975, Appl Microbiol, September p483-484). In each case a solution comprising 5% propylene oxide plus 70% isopropyl alcohol or 0.5% chlorhexidine or 2% Cetrimide was effective in destroying a bacterial spore suspension. However, while the use of propylene oxide has been recorded this is usually applied in the presence of alcohol (ethanol or isopropanol). Thus, the use of an alcohol as an additive to propylene oxide sterilisation with aldehyde cross-linked tissues (containing residual aldehydes) could result in elevated aldehyde levels, which in turn increases the calcification potential of these tissues and ultimately bioprosthetic failure.
Consequently, what is required is an efficient sterilization process which not only sterilizes chemical cross-linked collagen-based biomaterials, but also provides a convenient storage medium for the sterilized biomaterial.